1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor and an electrically-assisted power steering apparatus.
2. Discussion of the Background
A magnetostrictive torque sensor is provided in the electrically-assisted power steering apparatus for a motor vehicle. The electrically-assisted power steering apparatus is a support apparatus for assisting a steering force using a motor that operates together with a steering wheel when a driver drives a motor vehicle and operates the steering wheel. In the electrically-assisted power steering apparatus, the magnetostrictive torque sensor detects a steering torque induced on a rotation shaft (a steering axle) coupled to a steering wheel through a steering operation performed by the driver. The electrically-assisted power steering apparatus controls an assisting steering force output from a motor on the basis of at least the detected steering torque and a vehicle speed signal output from a vehicle speed sensor that detects the speed of the motor vehicle so as to reduce the steering force exerted by the driver.
A magnetostrictive torque sensor having upper and lower magnetostrictive members disposed on a surface of the rotation shaft so that the upper and lower magnetostrictive members have magnetic anisotropy in the opposite directions has been developed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2004-245636). If a steering torque is applied to the rotation shaft, a rotational moment is induced and the rotation shaft is twisted. Accordingly, the two magnetostrictive members deform. At that time, by the inverse magnetostriction effect due to the magnetic anisotropy, deformation of one of the magnetostrictive members is further increased and deformation of the other magnetostrictive member is decreased. The increase/decrease in deformation changes, for example, the impedances of detection circuits disposed on the peripheries of the two magnetostrictive members. By detecting the changes, the magnetostrictive torque sensor can detect the steering torque applied to the rotation shaft.
However, if the assisting steering force output from a motor is transferred to the rotation shaft via gears, a bending moment that acts to bend the central axis of the rotation shaft is applied to the rotation shaft in addition to the rotational moment about the shaft. The magnitude of the bending moment varies in accordance with the position in the axis direction of the rotation shaft. More specifically, the magnitude of the bending moment is 0 (zero) at the point of application of the bending moment, and the magnitude increases with distance from the point of application of the bending moment. Accordingly, bending moments having different magnitudes act on the two magnetostrictive members formed at different locations. Since each of the two magnetostrictive members detects the rotational moment and the bending moment as the same deformation, detection accuracy of the magnetostrictive torque sensor for detecting the rotational moment decreases. This is because bending moments having different magnitudes act on the two magnetostrictive members and, therefore, even when the difference between the outputs (e.g., the impedances) of the detection circuits corresponding to the magnetostrictive members is obtained, the outputs for the bending moments cannot be canceled out. Accordingly, in order to cancel out the outputs for the bending moments, a technique for setting the geometric moments of inertia at the points at which the two magnetostrictive members are disposed to different values has been developed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2008-216162).